Retrofocus-type wide-angle camera lens

ABSTRACT

A retrofocus-type wide-angle camera lens is provided in which the back focus is especially long compared to the composite focal length. The lens comprises a front lens group having a negative composite focal length and a rear lens group consisting of a cemented positive lens, a negative single lens, a composite positive meniscus lens, and a single positive lens. The lens thus provided has an aperture ratio of F/2.8 and a viewing angle of over 80* with various aberrations, coma, curvature of the field and distortion highly corrected.

United States Patent =1 sinner-noon inventor Yoshiyuki Shimizu 1-68,Tamagawa Yohga-machi Setagaya. ku, Tokyo, Japan Appi. No. 888.041

Filed Dec. 24. 1969 Patented Nov. 23. 1971 Continuation-impart ofapplication Ser. No. 611,206, Dec. 27, 1966, now abandoned. Thisapplication Dec. 24, 1969. Ser. No. 888,041

RETROFOCUS-TYPE WIDE-ANGLE CAMERA LENS 2 Claims, 2 Drawing Figs.

U.S. Cl 350/214 Int. Cl G02b 9/64 Field oiSearch 350/214, 2 l 5 [56]References Cited FOREIGN PATENTS 83.412 6/l964 France 350/2l4 l.396.9073/1965 France 350/2l4 Primary Examir|er.lohn K. Corbin A!!0rne vAnton J.Wille ABSTRACT: A retrofocus-type wide-angle camera lens is provided inwhich the back focus is especially long compared to the composite focallength. The lens comprises a front lens group having a negativecomposite focal length and a rear lens group consisting ofa cementedpositive lens :1 negative single lens, a composite positive meniscuslens. and a single positive lens The lens thus provided has an apertureratio of F/lil and a viewing angle of over 80 with various aberrations,coma. curvature ofthe field and distortion highly corrected.

RETROFOCUS-TYPE WIDE-ANGLE CAMERA LENS This application is acontinuation-impart of my copending application bearing Ser. No. 611,206filed on Dec. 27, 1966, now abandoned.

This invention relates to a retrofocus-type wide angle camera lens.

in a single lens reflex camera, the back focus of the objective lens isrestricted because of the operation of the reflecting minor, and it isnecessary that the back focus (B,f) of the objective lens of, forexample, a 35 mm. camera should be longer than about 35 mm. Under suchrestrictions, it has been difficult to obtain a super wide angle lensbecause distortion and coma aberration are increased as the angle ofview is increased although a retrofocus type objective lens has hithertobeen used in order to obtain a wide angle objective lens whose focallength was shorter.

On the one hand, the increase in spherical aberration is not seriousinsofar as a wide angle lens is concerned, but the increase ofastigmatism and distortion is detrimental to the quality of the image,and therefore the correction of these aberrations must be solved.

The object of the present invention is to provide a retrofocus-type wideangle lens in which astigmatism and distortion are highly corrected.

In accordance with the present invention, there is provided aretrofocus-type wide angle camera lens in which the back focus isespecially long compared to the composite focal length, an apertureratio of F/2.8 and a viewing angle of over 80 being provided and inwhich the various kinds of aberrations, in particular, coma, curvatureof field and distortion are highly corrected.

Other objects, advantages and features of the present invention willbecome more apparent from the following description of an illustrativeembodiment in conjunction with the drawing in which FIG. 1 is across-sectional view of retrofocus-type wide angle lens according to thepresent invention; and

FIG. 2 shows the various aberration curves of the lens illustrated inFIG. 1.

As viewed from the direction in which light rays are incident, L is anegative single meniscus lens with the convex surface directed towardsthe object, L is a positive single lens, and L is a negative meniscuslens with the convex surface directed towards the object, the threelenses comprising the front group of the lens, the composite focallength of which is negative. Separated by a comparatively large airspace d, from the lens L is a cemented positive element comprising apositive lens L and a negative meniscus lens L Separated by air spaceat, is a negative lens L, followed by a composite meniscus lens formedby a negative lens L and a positive lens L,,. A single positive lens Lcompletes the rear group.

The lens arrangement is such that the following conditions are met:

Where d and its subscript designate distance along the optical axis andn and its subscript designate the refractive indices designated on thedrawing and hereinafter fully set forth. The present invention ischaracterized by the above mentioned conditions, and the gist of thepresent invention, resides in the fact that the composite focal lengthof the front group lenses L, to L is made negative to form a virtualimage of the object, the virtual image being formed at an indefinitedistance by means of the group of lenses L. to L, (Tessa-type lens isprovided in the opposite direction, and single convex lenses are madeinto a cemented lens of L and L1,). By means of lens L,,,

the real image is fon-ned at a predetermined position. in order toobtain an image whose aberrations are highly corrected it is necessaryto correct various aberrations of the first virtual image, and for thatpurpose, the correction of chromatic aberration is carried out byconstituting lens L, a negative lens. Other aberrations such asspherical aberration, coma, image curving, distortion, and the like, arenot highly corrected, but these aberrations are sufficiently correctedby the rear group of lenses.

The lenses L L L L L and L are lens groups having a positive refractivepower and the groups of lenses L to L, has a shape and refractive powernearly equal to that of the known Tessa, or triplet lens type arrangedoppositely. This aims to focus at infinity the virtual image formed bylenses L, to L through the aid of lenses L to L The lens L, gives apredetermined focal length to constitute the whole lens system.

A Tessa-type lens is principally a symmetrical lens type although itssymmetry is somewhat deformed. The central negative lens is provided forcorrecting spherical aberration and axial chromatic aberration andastigmatism. For this purpose, it is most effective to place it at thecentral position, since the negative lens if located at the centralposition, has little effect on distortion and magnification chromaticaberration. Lenses L to L adopt this idea of the Tessa-type and this isexpressed as 0.5d d,, 2d,,. With this condition, the negative lens Lshould be placed substantially intermediate of the positive lens groupsL L; and L L in other words, the condition l is effective for producinga plane image, and it is the condition for placing the lens L almost inthe middle between the composite lens L L and composite lens L L When soarranged, the main light rays pass through almost the center of lens L(near to the optical axis), minimizing the adverse effects ofastigmatism, at the same time constraining spherical aberrations. Whensuch conditions herein proposed is changed, the main light rays passthrough the periphery of L (the marginal portion of lens), and not onlyastigmatism but also the spherical aberration is increased.

The object of the condition (2) is to correct distortion and theinternal coma generated by the oblique light rays, i.e., the light rayscoming into the system from the outside of the main light rays of thelight coming in at a predetermined angle against the lens, suchaberration being experienced in the lens system of this kind. In otherwords, surface r, of lens L, is formed in the negative curved surface,and therefore the difference of the angle of incidence caused by thedifference of the height of incidence of said oblique light rays,becomes larger. Thus the oblique light rays are greatly refracted andthe internal coma generated by the front group of lenses L, to L; can besufficiently corrected. If the condition is reversed, the internal comagenerated in the front group of lenses is increased, and correction bythe rear group of lenses becomes difficult.

Lenses L,, L and L constitute a lens group having a negative refractivepower. However, when the position of the entrance pupil is behind thelens group, the negative lens has a tendency to coma aberration. ln thisinvention, the negative lens group of L L and L produces internal comaaberration, which must be compensated. Condition (2) was determined forcorrecting the internal coma produced relative to the light raysincident at a position far away from the optical axis. The stop of thelens system according to this invention is behind the lenses L, and L sothat when the light rays pass through r,,, the light rays receive largerefraction due to the negative radius of curvature of r, and refractiveindexes n n, so as to approach to the main light rays. Since theinternal coma aberration of the light rays incident externally of the'main light rays is caused by the refraction of said light rays in thedirection deviating too much from the main light rays, theabove-mentioned condition plays a role to compensate fo such comaaberration.

The object of the condition (3) is to correct the internal comagenerated by the aslant light rays, i.e., the light rays coming into thesystem from inside of the main light rays coming in at a predeterminedangle against the lens. In other words, the angle of incidence of theaslant light rays onto surface r,, of lens L, is larger than the angleof incidence of the aslant li ht rays coming onto surface r, of lens L,and the refraction by the surface r,, becomes larger than the refractionby the surface r However, when the direction of the sign of inequalityis reversed while keeping the sum of the refractive powers of r and r,,constant, the change of Petzvals sum is small, and therefore theeffectiveness in correcting astigmatism is decreased. The difference ofrefraction caused by the difference of the height of incidence of thelight rays passing through lens L is further decreased and therefore itis useless for the correction of the aberration.

Condition (3) is then to compensate the internal coma aberration of thelight rays incident at the position nearer to the optical axis than themain light rays, in this invention, the stop is positioned intermediateof lenses L, and L,, so that the light rays incident intemally of themain light rays will pass lens L, externally of the main light rays andthe height of said light rays passing through lens r,, becomes higherthan that of the light rays passing through the lens r Since r,, has apositive radius of curvature, the negative lens L, should bend towardthe image field side, so that the light rays receive a large refractionby r, to depart from the main light rays. This results in the correctionof the internal coma aberration. This effect is further assured by thecondition (I), i.e., 0.5d d,, 2d which assures passage of the main lightrays through substantially the central portion of the lens L The lightpath thereof is not effected too much by the bending of the lens L Theobject of the condition (4) is to attain the same object and effect ofthe condition (3). Namely, surface r,, of lens L corrects the internalcoma of the light rays coming into the system from inside of the mainlight rays of the light flux coming in at a predetermined angle, but theinternal coma is further corrected by the surface r between lenses L,,L,,. This correction is carried out in such a manner that it increasesthe refractive index n, of the negative lens L and decreases therefractive index n, of the lens L whereby the curving in the positivedirection generated on the sagittal image surface is compensated in thenegative direction. If the direction of the sign of inequality isreversed, the angle of incidence of the oblique light rays becomessmaller and becomes useless for the correction of the internal comagenerated in the front groups.

Condition (4) is thus determined for achieving two purposes. As is wellknown, it is necessary to make the refractive index n, of the negativelens L, larger than n, of the positive lens L, so as to curve thesagittal image plane toward the object side, i.e., negative. The otherpurpose is to compensate the internal coma aberration due to r,, of thelight rays incident internally of the main light rays. The result ofthis condition is substantially the same as for r,,, i.e., the cementedsurface oflenses L, and L The image of the object is formed at almostindefinite distance through lenses L, to L when the above givenconditions are satisfied, but it is not always necessary to form theimage of the object at the indefinite distance. When the absolute valueof the composite focal length of the lens system from L, to L is lessthan twice the composite focal length of the whole lens system, and isin the positive, the composite focal length of the lens system from L toL becomes shorter, and the correction required becomes excessive, andthe correction of aberrations difficult. On the other hand, when it is anegative value, it would be necessary for lens L to have remarkablygreat refractive power. This also brings about poor balance, and thecorrection of aberration becomes difficult. Therefore, the compositefocal length of the lens group from L, to L, should be preferably morethan twice as much as the composite focal length of the whole lenssystem, and the virtual image or the real image fonned at the indefinitedistance can be formed as the real image at the rear of the lens bymeans of the lens L As a matter of fact, in addition to the conditionsgiven above, various kinds of conventional means for correctingaberrations are employed, and as a result, a retrofocus-type wide anglecamera lens, having an aperture ratio of F/2.8, an angle of view of overthe back focus of which is over 1.5), and which'has been highlycorrected for coma, image-surface curving and other aberrations, hasbeen obtained.

In the above examples, r,r,...., represent the radii of curvature oftherespective elements, d,d,...., the axial thicknesses of the respectiveelements and the air spaces of the adjacent elements, n n m, therefractive indices of the respective elements and vd..., the Abbenumbers for the glasses of respective elements.

wherein r,r represent the radii of curvature of thei" respectiveelements; d d represent the axial thicknesses of the respective elementsand the air spaces of the adjacent elements; n,n represent therefractive indices of the 5 Se1del aberration coefiiment-s of Example Iare g1ven below:

1' I II III-IV Iv v 0.0151218 0. 0243690 0. 0735414 0. 1024196 0.3100342 1. 2116956 0. 1349224 0. 0564436 0. 4556300 0. 0695353 0.19-10116 0.1327503 0. 1316670 0. 1029323 0. 1320126 0. 0020366 0.0064990 0. 0709364 0. 1375674 0. 5740902 0. 2697354 0. 1225240 0.1112396 0. 3619426 0. 1643776 -14. 7332946 1. 9467302 -0. 512703 1.0917724 0.1437694 9. 2949376 1. 0133750 0. 250297 0. 4639270 0.0600550 1. 1427144 0. 3703233 0. 2400244 0. 17344773 -0. 0562194 2.4595612 -1. 1533944 1 091152 0. 7303013 3675034 -0 6247514 0.5723000 -01942446 -0.5212666 0 3617100 -1 9773536 -0. 9953433 -1 001302 0. 3265912-0 4159774 0.0175012 0. 0533446 0. 3957122 0.1099196 0 3695372 -16245310 -0. 5333596 0. 3502104 0. 2235402 -0 0733396 4. 5633924 0.6047463 0 1601082 0.5395192 -0 0714194 -0. 0005906 0. 006630 0. 1433556-0. 0211022 0. 2363340 4. 4003996 -0. 7133146 0. 2343393 0. 3370592 -0.0550592 0. 9417450 -0. 3215354 0. 0496962 0. 1741644 0. 2343562 inaccordance with the present invention described it is possible toproduce a small, light-weight, retrofocus-type wide angle camera lenswherein the angle of view is over 80, and F is 2.8, and the back focusB.f is over 1.5 times more than the composite focal length. and in whichthe various kinds of aberrations are highly corrected over the wholeangle of view. When the lens system of the present invention is mountedin a single lens reflex camera. it is not necessary to elevate thereflecting mirror in advance. The lens system of the present inventioncan be incorporated into a camera in the same manner as a conventionallens system.

What is claimed is:

l. A retrofocus-type wide angle camera lens system, of which thenumerical data is as follows:

Focal lengthf=l .0

Back Focal Length B.f=l .5597

Angle ofField 2"=84 Aperture Ratio F:2.8

d,.=0.0042 r m-8.0000

respective elements; and vd. represent the Abbe Numbers of therespective elements.

2. A retrofocus-type wide angle camera lens, of which the numerical datais as follows:

Focal Length f=1 .0 Back Focal Length B.f=l .5032 Angle of Field 2=80Aperture Ratio F:2.8

ml vd r,=+2.0873

l =0. l !1 =I.6Z l4l 60.3 7 4071142 1I \=0.09Z =l.5ll9l3 6L2 ,.,=+0.4s20

1I,=0.200 n .=1.57s01 41.3 03400 a.1=0.0s4 11.=1.s52411 59.9 r,=|.470'-)11,..=0. 11.1.5131: 59.0 HEW-0.9280

d =(l.OZ8 Il =|.7847 26.1 rl3=+l.l768

d,,=o.104 n,=l.76684 46.2 r,. =-0.9516

wherein r,r .represent the radii of curvature of the respectiveelements; d,d 1 .represent the axial thicknesses of the respectiveelements and the air spaces of the adjacent elements; n n .represent therefractive indices of the respective elements; and vd. .represent theAbbe Numbers of the respective elements.

I? i i i t A

1. A retrofocus-type wide angle camera lens system, of which thenumerical data is as follows: Focal length f 1.0 Back Focal Length B.f1.5597 Angle of Field 2 Omega 84* Aperture Ratio F:2.8 r1 +2.5000 nd vdd1 0.1125 n1 1.62041 60.3 r2 +0.8958 d2 0.2625 r3 +3.7500 d3 0.2292 n21.62041 60.3 r4 -3.7500 d4 0.0042 r5 +1.2500 d5 0.1125 n3 1.62041 60.3r6 +0.4583 d6 0.4083 r7 +1.0521 d7 0.1667 n4 1.62004 36.3 r8 -0.7742 d80.1250 n5 1.51823 59.0 r9 -1.4542 d9 0.1042 r10 -3.5417 d10 0.2208 n61.7847 r11 +1.3500 d11 0.0979 r12 -5.0000 d12 0.0417 n7 1.7847 26.1 r13+1.1729 d13 0.2250 n8 1.62041 60.3 r14 -0.8333 d14 0.0042 r15 +8.0000d15 0.1458 n9 1.744 44.9 r16 -1.9423 wherein r1r2. . . . . represent theradii of curvature of the respective elements; d1d2. . . . . representthe axial thicknesses of the respective elements and the air spaces ofthe adjacent elements; n1n2 . . . . represent the refractive indices ofthe respective elements; and vd. . . . represent the Abbe Numbers of therespective elements.
 2. A retrofocus-type wide angle camera lens, ofwhich the numerical data is as follows: Focal Length f 1.0 Back FocalLength B.f 1.5032 Angle of Field 2 Omega 80* Aperture Ratio F:2.8 nd vdr1 +2.0873 d1 0.180 n1 1.62041 60.3 r2 +0.7842 d2 0.300 r3 +4.6000 d30.552 n2 1.6228 56.9 r4 -2.9420 d4 0.004 r5 +1.1160 d5 0.092 n3 1.5891361.2 r6 +0.4520 d6 0.232 r7 +0.8400 d7 0.200 n4 1.57501 41.3 r8 -0.8400d8 0.054 n5 1.56248 50.9 r9 -1.4709 d9 0.088 r10 -40.000 d10 0.160 n61.51823 59.0 r11 +0.9280 d11 0.112 r12 -0.9200 d12 0.028 n7 1.7847 26.1r13 +1.1768 d13 0.180 n8 1.6679 55.5 r14 -0.7440 d14 0.004 r15 -8.8000d15 0.104 n9 1.76684 46.2 r16 -0.9616 wherein r1r2. . . . .represent theradii of curvature of the respective elements; d1d2. . . . .representthe axial thicknesses of the respective elements and the air spaces ofthe adjacent elements; n1n2. . . . .represent the refractive indices ofthe respective elements; and vd. . . . .represent the Abbe Numbers ofthe respective elements.